Porsche’s rear-engined icon may be in the twilight phase of its 991 model cycle, with an all-new car due in less than two years, yet fact is it’s never been more primed for war than in this 991.2 guise and specced to ultra-desirable GTS level.

But before we delve into a full rundown of our combatants, a quick disclaimer: This two-car battle began as a stoush between a couple of Stuttgart-born stormers, but evolved into more of a tag-team tussle when our first Porsche – a PDK-equipped GTS coupe which starts life at $286,390, but was specced up to $339,230 – developed a rare electrical issue. In its place came a Targa 4 GTS, bringing all-wheel drive, an electrically folding roof panel, and a base price of $316,000. Our example was comparatively tamely specced to $342,830.

Which explains why there are two different Porsche 911 GTS models on these pages, and the challenge being flung at the Mercedes-AMG GT C Roadster. Through no premeditation of our own, Porsche effectively had two cracks at finding the sweet spot in the GTS line-up to take down Merc’s open-top head-kicker.

It’s worth a quick recap to understand the GT C’s positioning here, as the uninitiated may confuse the ‘C’ bit as indicating cabrio or convertible, but this isn’t the case. The ‘C’ denotes the spec level sitting a rung above GT S (so two grades above the base GT), but below the ultra-focused AMG GT R. The C (which will also arrive in coupe form next year) gets the wider rear track of the GT R, along with 57mm wider guards to house it, as well as the flagship’s rear-wheel steering and electronically controlled limited-slip differential. The GT C’s 4.0-litre twin-turbo V8, too, is ramped up to 410kW/680Nm, so not far short of the 430/700Nm of the full-house GT R.

Being funnelled purely through the rear treads – admittedly vast 305/30R20 Continentals – may have even hardened power junkies deeming 410kW/680Nm as ‘adequate’; factor in streaming wet Victorian roads for our two test days and we may revise that to ‘adequate plus a bit’.

But it would be doing the GT C a disservice to jump straight in and spout about its apocalyptic straight-line thrust, and its ability to make your mouth form ‘holy crap’ shapes when that V8 is bellowing up at the business end. What also needs to be appreciated is how linear, docile, and tractable it is when you want to just trundle around. You can still be the first away from the lights and remain aurally unobtrusive without ever exceeding 2000rpm; just modulate the throttle slightly to prompt early upshifts from the seven-speed dual-clutch gearbox and surf along on that monster undertow of torque.

When the road opens and your restraint evaporates, though, the GT C is capable of acceleration that is vicious, loud, and seemingly unending until you’re into heavy triple digits. There’s something vaguely reminiscent of an offshore powerboat in the way the vast prow of a bonnet that stretches out ahead of you rises slightly when right foot meets firewall, the rear end scrabbles for traction, and the switchable exhaust allows full thunder from the oversized oval pipes.

The note doesn’t have the same crisp, unadulterated snap and crackle of the old atmo 6.2-litre unit in the SLS Roadster – there’s no escaping the fidelity-muting nature of the two turbos mounted in the engine’s vee – but the shove from low in the rev range and the ferocity of the mid-range is not just in a different ballpark to the old engine, it’s in a different suburb.

All of which lulls you into thinking that the Porsche’s downsized flat-six, giving away two pots and around a litre of capacity, is going to struggle to compete with such overt V8 fire and fury.

Maybe in theatre, but not in real-world performance. There’s a zingy, elastic quality to the 3.0-litre Porsche engine that is a triumph of maintaining throttle response and driver connection against the opposing demands of turbocharging. Outputs in GTS spec are up 22kW and 50Nm over the Carrera S, and yes, you can feel it.

Okay, an ardent lover of the old 3.8-litre atmo six (err, that would be me) might mention that the really tingly yowl of that engine has been neutered slightly, and the necessity to wring it to redline has been eroded just a little. But the now-bulging mid-range translates to serious real-world punch, firing the car out of tighter corners with the sort of sting that would have had you in a holding pattern for a few moments while the old engine climbed into its boogie pants.

So, factor in the Porsche’s 190kg weight advantage over the GT C, plus the power-down benefits brought about by the rear-engined weight distribution, and you have a compact sports car that remains indecently rapid no matter what the weather conditions. At the test strip, the GTS coupe bolted like it had been smacked in the back by a roadtrain, hooking up virtually instantly and hitting 100km/h in 3.4sec, two-tenths quicker than its claim. It smoked through the 400m trap in 11.4sec, a mere 0.2sec slower than a 911 Turbo we tested recently, and cementing Porsche’s position that GTS owns the zone above Carrera S, but is more liveable and easy-going than the GT3.

On a wet strip, the big AMG couldn’t match that, delivering a less-than-awesome 4.0sec. In the dry, though, and aided by its launch-control function, which allows you to adjust the pre-set engine revs up or down via the shift paddles, we have no doubt it would deliver on its 3.7sec claim.

So, despite differing approaches, both achieve similarly scalding performance. More significant are the differences in the way each car interacts with its driver, and method in which each goes about doing a demolition job on a quiet back road. The Porsche instantly feels much more compact and intimate; it seems a little ironic that when the 991 generation first launched back in 2011, there were moans that Porsche had taken the 997 design and stuck it on a photocopier and hit the ‘120 percent enlarge’ button.

Now, it feels ideally sized; you and your passenger sit with just enough shoulder room between you, the driving position is perfect, the glasshouse giving great visibility in all directions. The Merc-AMG, by comparison, feels big; it sits you lower; legs ahead and staring down the long, slightly phallic bonnet. It also positions the gear selector too far back and puts a bunch of buttons on the console where your left elbow naturally falls.

Steering, too, shows some radically different thinking at work. The 911’s system has been further improved with this 991.2 update, and to find an electrically assisted system as feelsome, natural, ideally weighted, and giving you such confidence in the front end, well, you’d open the Porsche catalogue in the Boxster/Cayman section. Again, Porsche enthusiasts went all emoji-sad-face when the axe fell on the 997’s hydraulically assisted rack, and lamented that the writhing, analogue feel had gone for good. The current car may have a facsimile of that set-up, but it’s a bloody accurate one, and hard to argue if it’s notably inferior.

It’s a tactile connection the GT C has no answer to, at least not in moderate driving on sweeping roads that require only small inputs. Here the AMG’s wheel feels a little aloof, not really interested in engaging your sense of touch; more intent on filtering out the feedback of surface irregularities that the Porsche is happy to pass on. Only when the corners start to tighten and the demands for more lock are made does the AMG’s steering start to reveal itself, and the speed of the rack, the consistent weighting and the instant, incisive response from the front axle can be appreciated.

Yet there’s still a layer of communication missing that’s always there in the Porsche. In the wet, the Merc takes a ‘trust me, I’ve got this’ approach as you go probing for the limits of adhesion, rather than taking you by the hand and actually showing you. There’s no questioning the GT C’s depth of dynamic ability, but you need to have vast confidence in your own abilities, because the car isn’t all that keen to reveal everything until it’s actually pushing the nose or slithering its giant booty. If that isn’t enough to fully focus your attention, just throw some bumps into the mix.

The Comfort setting continues the AMG in-joke of being code for ‘ultra-firm’, while Sport only works on uncommonly smooth roads, leaving Sport Plus effectively limited to a track. This iron-fisted approach to spring and damper tuning makes sense when you hammer the car through fast undulations, where the hand-of-God body control can be appreciated; it’s less welcome in terms of the toll it takes on comfort and the car’s broader GT abilities.

The comfort issue isn’t helped by seats with super-firm, thinly padded bases, and insistent road roar on coarse-chip coming from the huge rear tyres. It’s a shame because the cabin’s roof-up insulation is exceptional, with wind noise and ambient racket beautifully zipped out.

The 911 suffers from even louder coarse-chip roar – surely the engineers can fix this for the next generation? – but is not just a more connected and tactile hard-driving experience, it’s also the more comfortable tourer, provided you’re okay with a firm ride and circumspect with the options list. The GTS coupe cops a 20mm drop in ride height; ours was fitted with optional Dynamic Chassis Control, which takes another 10mm out of the ride height in conjunction with active anti-roll bars. In this spec, the coupe delivers a ride that was marginal on bumpy backroads.

In the wash-up – literally, in terms of our driving conditions – we need to cut the Merc a break, because we know the constantly wet roads during our test didn’t play to its strengths, and we know what the Roadster is capable of on smooth, dry tarmac from our time on American roads at the car’s launch. Its grip levels are neck endangering, the rear steering gives it an agility and pointiness that elevates it to a level above the GT and GT S, and that engine just keeps on making you shake your head and mumble expletives.

It’s also one of those ‘event’ cars that sucks people in and somehow makes them feel better for having pored over it. It’s the sort of car you wish someone had taken the 10-year-old you for a roof-down blast in, just to have blown your tiny mind and got you hooked via the octane vein even earlier.

It also works brilliantly with the roof down; praise that can’t be shared with the 911 Targa. Where the AMG minimises cabin turbulence and manages occupant comfort with a gilt edge, Porsche’s Targa drops the ball. Its airflow ‘management’ amounts to turning the rear of the cabin into a windsock, and the buffeting around your ears at highway speeds will have you quickly grabbing for the button to put the roof back where it belongs.

The AMG also has the ability to render the 911 near-invisible. This pair is like George Clooney with a bodyguard; people who strolled over for a chat and a drool during stops over the course of our two days were vaguely aware of ‘the other car’, but showed little interest and would have no recollection of it.

Which will probably be as relevant to 911 GTS owners as George Clooney’s star sign. They know they’ve bought the world’s greatest all-round sports car; a machine honed to near-perfection, with an unbeatable depth of engineering integrity and real-world useability. The sun may be setting on the 991-generation 911, but light still glints off the GTS’s blade.

Sign up here to receive the latest round-up of Wheels news, reviews and video highlights straight to your inbox each week.